1. Field of the Invention
The invention is related to the field of measurement systems, and in particular, to a system and method that use measurements from a Coriolis flow meter to determine the amount of proppant in a fracture fluid.
2. Statement of the Problem
Oil, gas, and other resources under ground are obtained by drilling a well. The well is drilled to a certain depth and cased in cement. The well extends through multiple zones in the ground that a drilling crew may wish to tap. To tap into a certain zone, the drilling crew fractures a portion of the casing in the desired zone. The fracturing process used could be hydraulic fracturing, pneumatic fracturing, or another type of fracturing. With the casing fractured, the drilling crew then pumps a fracture fluid into the fracture to keep the fracture open. The fracture fluid holds the fracture open while still being permeable. This enables the oil and gas to more easily flow through the fracture into the well-bore.
The fracture fluid is made up of a base fluid and a proppant. To make the base fluid, a Guar gum is added to water in a large tank. A mixer within the tank continually mixes the Guar gum and the water together to make the base fluid. When mixed, the base fluid has the consistency somewhat like molasses.
A proppant, such as sand, is then added to the base fluid in the tank to make the fracture fluid. The amount of sand added depends on soil type, soil conditions, and other factors. The mixer in the tank mixes the base fluid and the sand together to make the fracture fluid. The fracture fluid is then pumped into the well-bore to help keep the fracture open. The amount of the sand in the fracture fluid determines how well the fracture fluid is able to hold the fracture open.
Because the amount of sand in the fracture fluid is important, the drilling crew may want to measure the amount of sand added. This can be a difficult process because the fracture fluid is usually not made in a batch, but is continuously mixed. To determine the amount of sand in the fracture fluid, the drilling crew uses a nuclear densitometer to measure the density of the fracture fluid being pumped into the well-bore. A controller receives the density measurement from the nuclear densitometer and calculates the amount of sand added to the fracture fluid. The drilling crew can then adjust the amount of sand to a desired level. An example of a system for providing the fracture fluid is described below and illustrated in FIG. 1.
Unfortunately, there are problems associated with using nuclear densitometers. For instance, interstate and international transport of nuclear densitometers can be a difficult process considering the laws and regulations surrounding nuclear technology. There are also concerns for safe handling and transporting of the nuclear densitometers. The operators of the nuclear densitometers have to be certified or licensed by the proper regulatory agency. Such factors make nuclear densitometers undesirable to use.
Coriolis flow meters are used to measure the mass flow rate, density, and other information for fluids. Exemplary Coriolis flow meters are disclosed in U.S. Pat. No. 4,109,524 of Aug. 29, 1978, U.S. Pat. No. 4,491,025 of Jan. 1, 1985, and Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. Coriolis flow meters are comprised of one or more flow tubes of a straight or curved configuration. Each flow tube configuration in a Coriolis flow meter has a set of natural modes of vibration, which may be of a simple bending, twisting, torsional, or coupled type. Each flow tube is driven to oscillate at resonance in one of these natural modes of vibration. Fluid flows into the flow meter from a connected pipeline on the inlet side of the flow meter. The fluid is directed through the flow tube(s), and exits the flow meter through the outlet side of the flow meter. The natural vibration modes of the vibrating, fluid-filled system are defined in part by the combined mass of the flow tubes and the mass of the fluid flowing through the flow tubes.
As fluid begins to flow through the flow tubes, Coriolis forces cause points along the flow tubes to have a different phase. The phase on the inlet side of the flow tube commonly lags the driver while the phase on the outlet side of the flow tube leads the driver. Pickoffs are affixed to the flow tube(s) to measure the motion of the flow tube(s) and generate pickoff signals that are representative of the motion of the flow tube(s).
Meter electronics, or any other ancillary electronics or circuitry connected to the flow meter, receive the pickoff signals. The meter electronics processes the pickoff signals to determine the phase difference between the pickoff signals. The phase difference between two pickoff signals is proportional to the mass flow rate of the fluid through the flow tube(s). The meter electronics can also process one or both of the pickoff signals to determine the density of the fluid.
Unfortunately, Coriolis flow meters have not been used to measure the density of a fracture fluid. First, the fracture fluid is usually pumped down the well-bore through a large tube, such as an eight inch tube. Coriolis flow meters have not been built large enough to measure an eight inch stream. Secondly, most Coriolis flow meters have curved flow tubes. The erosive properties of sand through the curved flow tubes prevents the curved-tube Coriolis flow meter from being a viable option. The sand would damage the flow tubes in a matter of hours. For these reasons, Coriolis flow meters have not been used to measure the fracture fluid, and nuclear densitometers continue to be used.